1. Field of the Invention
The invention relates to a lift-type vertical axis wind turbine, and more particularly to a structure capable of enabling the wind mill to maintain relatively stable rotation speeds under wind speeds exceeding the rated wind speed by adjusting the angle of the supporting arms which hold the blades, thereby ensuring stable output from the vertical axis wind turbine.
2. Description of the Related Art
The followings are some terminologies concerning wind turbine technologies:
Wind speed: a mean value of wind speeds at a certain height in a 10 minutes period, and usually the mean value of wind speeds at 10 meters above grassland in a 10-minute-period for reference.
Effective wind speed: since the wind speed varies all the time, not all wind speed is able to make the wind mill to rotate or to rotate safely. Effective wind speed refers to the wind speed which can make the wind mill rotate safely.
Effective wind speed range: the wind speed that can make the wind mill rotate safely and the generator output normally. Upon designing wind turbines, the range between the cut-in wind speed and the cut-out wind speed is called effective wind speed range, e.g., between 6 and 20 m/s.
Cut-in wind speed: the minimum wind speed at which the wind mill starts to rotate and power is generated.
Rated wind speed: the wind speed at which the rated output (nameplate capacity) is reached.
Cut-out wind speed: the wind speed above which the wind mill is shut down to avoid damage.
Speed governor: the rotation speed of wind mills varies with the wind speed, and a device that makes the wind mills rotate around the rated rotation speed is called a speed governor, which works only when the wind speed exceeds the rated wind speed.
Vertical axis wind turbine: wind turbines can rotate around either a horizontal or a vertical axis, therefore wind turbines can be divided into horizontal axis wind turbines and vertical axis wind turbines, and wind turbines that rotate around a vertical axis are called vertical axis wind turbines.
Wind mill: the wind mill of the vertical axis wind turbine includes a plurality of supporting arms and blades, the blades are connected to the vertical shaft via the supporting arms.
Vertical axis wind turbines can be divided into drag-type and lift-type, which are different in working principles and configurations. The blades of the drag-type wind turbine may be of a cup, a semi-sphere, a half bucket, or even simply a flat board. FIG. 1 shows a drag-type vertical axis wind turbine. The blade is of a half bucket, and when the wind blows the wind mill, the pressure exerting on the blade at the D1 point is two times that exerting on the blade at the D2 point. Since the pressures at D1 and D2 are different, the wind drives the wind blades to rotate around the hub clockwise. Therefore, the most distinct feature of the drag-type wind turbine is that the wind drives the blades to move from high pressure point to low pressure point, and thus the wind mill rotates from high pressure point to low pressure point.
To improve the efficiency of the drag-type wind turbine, i.e., to increase the pressure difference between the D1 and D2 point of the blades, various measures have been tried, e.g. using more blades, adjusting the shape of the blades, and so on. The most widely-used method is to adjust the angle of the blades when the blades rotate in the lower section of the wind mill. As shown in FIG. 2, assume the angle is 0° at the D1 point, when rotating to the D2 point, the blade is modified to have an angle of 90°. There are many measures to adjust the angles of the blades, but no matter what measures are employed, the wind energy utilization efficiency of the drag-type wind turbine can never exceed 2/27. The following, ignoring the resistance caused by the drag-type wind turbine blades in the lower section, shows how the conclusion is reached.
Assuming the length of each blade is H, and the width thereof is R, the angular velocity of the wind mill rotation is ω, the radius of the wind mill equals the blade width R, the wind speed is V, and the density is ρ; suppose the wind-to-blade pressure have the maximum value, and regardless of the blade resistance in case the projected area is in the minimum, the blade with the biggest projected area equals to a board with a width of R and length of H.
Total wind energy on the projected area of wind mill is:
  E  =                    1        2            ⁢      ρ      ⁢                          ⁢              V        3            ⁢      S        =                            1          2                ⁢        ρ        ⁢                                  ⁢                  V          3                ⁢        2        ⁢        RH            =              ρ        ⁢                                  ⁢                  V          3                ⁢                  RH          .                    
The airflow-to-blade speed is: v=V=ωR.
The airflow-to-blade pressure is:
  p  =            1      2        ⁢                  ρ        ⁡                  (                      V            -                          ω              ⁢                                                          ⁢              R                                )                    2        ⁢          RH      .      
The output is:
      P    ⁡          (      ω      )        =                    T        ⁢                                  ⁢        ω            ≤              pR        ⁢                                  ⁢        ω              =                            1          2                ⁢                              ρ            ⁡                          (                              V                -                                  ω                  ⁢                                                                          ⁢                  R                                            )                                2                ⁢        RHR        ⁢                                  ⁢        ω            =                        1          2                ⁢                              ρ            ⁡                          (                              V                -                                  ω                  ⁢                                                                          ⁢                  R                                            )                                2                ⁢                  R          2                ⁢        H        ⁢                                  ⁢                  ω          .                    
Finding the extreme values and we get: when ωR=0=V, P gets the minimum value, and when ωR=⅓V, P gets the maximum value, i.e. the biggest output of the drag-type wind turbine is achieved when the linear speed of the rotation of the wind mill is ⅓ the wind speed. Substituting ωR=⅓V, we get the maxima of output is
            2      27        ⁢    ρ    ⁢                  ⁢          V      3        ⁢    RH    ,ignoring the resistance in the other section, and in accordance with the definition of wind energy utilization efficiency, we get:
                    η        =                              Output            ⁢                                                  ⁢            of            ⁢                                                  ⁢            wind            ⁢                                                  ⁢            mill                                total            ⁢                                                  ⁢            wind            ⁢                                                  ⁢            energy            ⁢                                                  ⁢            on            ⁢                                                  ⁢            projected            ⁢                                                  ⁢            area            ⁢                                                  ⁢            of            ⁢                                                  ⁢            wind            ⁢                                                  ⁢            will                                                  =                              P            E                    =                                                                      2                  27                                ⁢                ρ                ⁢                                                                  ⁢                                  V                  3                                ⁢                R                ⁢                                                                  ⁢                H                                            ρ                ⁢                                                                  ⁢                                  V                  3                                ⁢                R                ⁢                                                                  ⁢                H                                      =                                          2                27                            ≤                              8                ⁢                                  %                  .                                                                        
To address the low efficiency of the drag-type vertical axis wind turbine, the lift-type vertical axis wind turbine has been created. FIG. 3 shows a lift-type vertical axis wind turbine, whose surfaces are of different curves. When the wind blows on the blades, the wind speeds at the inner surface and the outer surface of the blades are different because of the different shapes of blade surfaces and the setting angle of the blades. Therefore, wind speed difference exists between the inner surface and the outer surface. Fluid mechanics tells us that when the flow speeds at the inner and outer surfaces are different, pressure difference, i.e., lift, is formed. When certain setting angles of the blades are used, the component force of the lift created by the pressure difference will create a torque surrounding the wind turbine hub, thereby making the wind turbine rotate.
FIG. 4 is a schematic drawing of the force analysis of a lift-type vertical axis wind turbine. Since the actual speed and direction of the airflow-to-blades is V2 (In FIG. 4, V0 represents the wind speed, and V1 represents the speed of wind-to-blades-rotation-direction), under such wind speed and direction, a resistance D parallel to the airflow and a lift L perpendicular to the airflow are formed in the blades. When the wind speed exceeds a certain value, the lift in the airfoil is far greater than the resistance D; therefore the blades are mainly driven by the torque produced by the tangential direction component L1 of the lift L. Therefore, the most distinctive feature of a lift-type wind turbine is that its blade section must be in the shape of curves (airfoil profile) and the setting angle is small. The airfoil usually is one of those in the prior art, or a new airfoil formed using different curves of conventional airfoils, or a new airfoil formed by a plurality of curves at least meeting second order derivable function, or a new airfoil formed by splines.
The scale of aerodynamic resistance formed during the blade rotation is related to the setting angle of blades. The bigger the setting angle, the bigger the resistance. Therefore, to achieve a better efficiency, the setting angle of lift-type wind turbine blades is usually small. To illustrate the importance of the setting angle of the blades to the efficiency of the wind mill, definitions regarding blade specifications are given below (see FIG. 5):
Leading edge (of the blade): the round end.
Trailing edge (of the blade): the sharp tail.
Chord: the line between the leading edge and the trailing edge.
Setting angle α: the angle between the chord and the tangent line that goes through the center of the blade, and α is a positive value clockwise, and a negative value counterclockwise.
Azimuth: the angle between the blade-center to axis-of-rotation line and the positive y axis line.
The scale and direction of the torque generated on the blades varies with different blade azimuths, and at certain azimuths, the direction of the torque even reverses. To increase the efficiency of the lift-type vertical axis wind turbine, many measures are put forward, for example, choosing the right airfoil, setting angles, chord length, and blade number. A more effective way is to alter the setting angle α while the blades are in different azimuths, so that the blades can obtain the biggest lift at any azimuth. The measures focus on altering the setting angle of the blades when the blades are in different azimuths during one revolution, making the blades obtain the best setting angles α. The blades, in one revolution, obtain numerous comparatively small setting angles α. The setting angle changes when the wind mill rotates, which makes the lift-type vertical axis wind turbine obtain enough torque in low wind speed and resultant slow speed of rotation. The purposes are to improve the self-starting ability and the efficiency of the lift-type wind turbine in comparatively high wind speeds.
It can be concluded that the drag-type vertical axis wind turbines are totally different from the lift-type vertical axis wind turbines, so are the measures to improve their efficiency.
For vertical axis wind turbines with a fixed blade setting angle, within certain wind speed ranges, the rotation speed of the wind mill is proportional to the wind speed. The higher the wind speed, the higher the rotation speed. The wind energy is proportional to the cube of the wind speed. When the wind speed increases form 10 m/s to 25 m/s, the wind energy increases by nearly 16 times. While the wind speed may change dramatically, each wind turbine has its rated wind speed, at which the wind turbine works best. When the wind speed exceeds the rated wind speed, the output of the wind turbine is desirable to maintain around the rated output to protect the generator and system from damage. This problem is solved by yawing (reducing the projected area of the wind turbine) in horizontal axis wind turbines.
Because the blade setting angle of the drag-type vertical axis wind turbine can be changed in a wide angle range unrestrained, the drag-type vertical axis wind turbine reduces the projected area of the wind turbine to achieve the same object. Measures reducing the projected area of the wind turbine to lower the output will not change the efficiency of the wind turbine. For example, the diameter of the wind turbine in high rotation speed is contracted to stabilize the output. In the extreme case, the wind turbine is contracted to a bucket shape.
The lift-type vertical axis wind turbine may reduce the projected area of the wind turbine to lower its output by hydraulic units and electronic control technology, but the configuration is complicated and costly, which makes it difficult to be widely used in small and medium wind turbines, and it will not be elaborated here. We can see from the equation for the wind turbine output
      P    =                  1        2            ⁢      ρ      ⁢                          ⁢              V        3            ⁢      SCp        ,to lower the output of the wind turbine, besides cutting the projected area S of the wind turbine, a preferred method to stabilize the output is by lowering the efficiency Cp of the wind turbine, thus protecting the turbine system from damage caused by strong winds at a lower cost.
In the prior art, to achieve a higher efficiency of wind turbines, the preferred blade setting angle is from 2° to 8°, and within such range, the efficiency changes only slightly, while out of the range, the efficiency of the wind turbine decreases rapidly. A restriction slot is used to restrain the changes of the blade setting angle within a narrow range. Furthermore, changing the blade setting angle can lower the efficiency of the lift-type turbine when the wind speed exceeds the rated speed to stabilize the output under high winds conditions, in which the blade setting angle changes within a narrow range through a flexible component. However, in the prior art, the changing range of the blade setting angle has not been disclosed, and the flexible component needs further improvement.